Free cutting alloys



Patented Apr. 13, 1937 FREE CUTTING ALLOYS;

Louis W. Kemp! and Walter A. Dean, Cleveland, Ohio, assignors to Aluminum Gompany of America, Pittsburgh, Pa.,. a' corporation. of

Pennsylvania No Drawing. ApplicationDecember 28, 1935;

Serial No. 56,548

Claims.

The invention relates to aluminum base alloys and is particularly concerned with aluminum base alloys containing copper, tin and cadmium.

Aluminum base alloys containing between 3 5 per cent and 12 per cent of .copper have been widely used heretofore. The copper imparts good casting characteristics and increases the tensile strength, yield strength, and hardness of the alloy. In the lower portion of the copper range the alloy may be mechanically deformed by the well known commercial processes such as rolling, forging, drawing, or extrusion. Alloys'containing more than about 6 per cent of copper are generally used in the cast condition. Throughout the 15 entire range of 3 to 12 per cent copper, however, the alloys are susceptible to variations and improvements in their physical properties by thermal treatments.

The physical properties of the foregoing type of aluminum-copper alloy in the solution heat treated and aged conditions are greatly improved by the addition of from 0.005 to 0.1 per cent of tin as described in co-pending application No. 606,756, and U. S. Patent No. 2,022,686, issued December 3, 1935. The small amount of tin is particularly useful in increasing the yield strength of the heat treated and aged alloy. The use of tin in the aforesaid proportions does not substantially change the working characteristics of the alloywhereas special precautions must be taken to roll or forge alloys containing a larger amount of tin. The addition of from 0.05 to 2 per cent of cadmium to the aluminum-copper-tin alloy described above serves to supplement the effect of tin and to improve the corrosion resistance of the alloy. Because of the high strength which is attainable in these alloys when heat treated and aged, they are generally used in this condition. Our invention which is described hereinbelow, is

directed to improving the machining quality of such alloys.

There are some applications wherein aluminum-copper-tin-cadmium alloys as hereinabove disclosed might be conveniently and profitably used except for an inherent disadvantage which militates against their use in the production of certain articles requiring exacting machining operations. Mechanical cutting operations such as drilling, shaping, or lathe-cutting are successfully carried out only by using certain precautions which increase the cost of production and perhaps favor the choice of another metal or alloy which can be machined more readily but which is not so desirable in other respects, as for example, in physical properties. When alloys are dlfllcult to machine this disadvantage becomes evident, in many cases, through rapid wear of the cutting tool edge, so that frequent-tool re-sharpening is required. Despite continual lubrication the machined surface is rough and irregular,

and the chip has a tendency to form a continuous curl or spiral which often fouls the tool or the moving parts ofthe machine. It is immediately nevertheless desirable to improve the machining properties to a still greater extent. Accordingl y an object of our invention is the provision of an aluminum base alloy containing from about 3 per cent to about 12 per cent of copper, from 0.005 to 0.1 per cent of'tin, and from 0.05 to about 2 per cent of cadmium which may be readily and economically machined.

Our invention resides in the discovery that the foregoing object is effected by the addition of one or more of the elements lead, thallium or bismuth. The aluminum base alloys to which these elements are addedin the proportions specified below, are known as free cutting or free machining alloys because they can be machined more rapidly than similar alloys without these elements and yet have as good or a better finished surface. After an extended series of investigations we have discovered that these three metals when added to aluminum base alloys, form a class of alloying elements by reason of their favorable effect upon the machining properties of these alloys. In

recognition of this effect we term lead, thallium and bismuth free machining elements. Although the presenceof one of these elements im-' proves the machinability, we have further discovered that the simultaneous presence of two or more of these elements is productive of an improvement in free machining characteristics" which is considerably greater than that caused by the presence of the same total amount of a single free machining element. For'examplathe addition of 0.5 per cent of lead and 0.5 per cent of bismuth to an aluminum base alloy containing about 5 per cent of copper, 0.05 per cent of tin and 0.2 per cent of cadmium effects a greater im-.

provement in machining quality'than doesthe addition of 1.0 per cent of either lead or bismuth singly.

These three elements, in conjunction with the.

cadmium in the base alloy, are, unique with respect to their effect on the machining characteris'tics of aluminum-copper-tin-cadmium alloys. It is a fortunate circumstance, therefore,

that they are also of relatively low melting point,

a fact which makes possible their addition to molten aluminum in the pure state, without the intervention of so-called "rich alloys. As a matter of fact, we, have observed that of all the metals whose melting point is lower than about 327 C., the melting point of lead, the three elements we 5 have selected, together with cadmium and tin,

are the only ones which are commercially suitable and that impart free cutting characteristics to an aluminum-copper-tin-cadmium alloy, but do not have undesirable effects on the fundamental physical properties of this base alloy.

The total amount of the added free machining elements should not be less than about 0,05 per cent since below this amount there is scarcely any advantageous effect; We have determined that a maximum limit of about 6 per cent total of one or more of the free machining elements disclosed issumcient for satisfactory commercial results, since although the free machining effect persists beyond this amount, certain of the other physical properties may be unfavorably affected.

'Aluminum-copper-tin-cadmium alloys containing one or more of the free machining elements lead, thallium and bismuth may be machined more rapidly, with less tool wear, less tool sharpening, better quality of chip and better machined surface than the same base alloys without the free machining additions, and in fact better than the same base alloys containing an equivalent total amount of a single free machining element.

Since aluminum base alloys containing from 3 to 12 per cent of copper have a'wide variety of applications we list several alloys each of which may be said to be preferred for a particular purpose. As an alloy for mechanical deformation we suggest an aluminum base alloy containing 6 per cent of copper, 0.05 per cent of tin, 0.25 per cent of cadmium, 0.5 per cent of'bismuth, the balance being aluminum. The machinability may be improved if 0.2 per cent bismuth and 0.2

40 per cent lead are employed in place of 0.5 per cent bismuth. For an alloy with excellent casting characteristics to be used in the worked condition we suggest an alloy containing 10 per cent of copper, 0.05 per cent tin, 0.5 per cent cadmium and a total of 3 per cent of free machining elements, the balance being aluminum.

For certain purposes, notably the improvement of tensile strength, hardness and grain structure, the alloys ,as hereinabove disclosed may be improved by the addition of one or more of the group -of elements composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium. From 0.05 to 1 per cent of any one of these elements may be used alone, but if more than one is employed the total amount should not exceed'about 2 per cent.

It is characteristic of the three elements, lead, thallium and bismuth, and of the tin and cadmium in the base alloy, that they form with aluco minum a series of alloys of limited liquid s0lubility. We have reason to believe that the free machining elements are the only elements which exhibit this characteristic, with the possible exception of several metals which are not regarded as having any commercial promise as additions to aluminum base alloy-1 Within the range disclosed. and claimed however the free machining elements may be added without unusual difficulty. "We suspect that this characteristic feature of the disclosed elements may be one of the significant factors which contribute to their free machining effect. We believe that this effect is further strengthened by distributing the free machining constituent relatively homogeneously throughout the solid matrix, since these free machining constituents are practically insoluble in the solid aluminum base.

The free machining alloys which have been described hereinabove may be subjected to the thermal treatments well known in the art to improve their strength and hardness. We have found that a solution heat treatment and subsequent aging does not impair the free machining quality of the alloys and in many instances the treatment even tends to improve this property. For many purposes a relatively high strength and hardness are necessary to the successful performance of the machined article and hence the alloy must be heat treated. This treatment is generally applied prior to the machining operation.

As hereinabove indicated the free machining elements, by reason of their low melting point,

may be added to the molten aluminum alloy in pure metallic form. However, since some difliculty may be encountered in introducing them in the higher percentages of our disclosed range we prefer to use the method which is more fully described in U. S. Patent No. 1,959,029, issued March 15, 1934. Briefly it involves heating the melt to a somewhat higher temperature than is customary, and vigorously stirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces the usual impuritiesfound in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incident to melting practice.

We claim:

1. An aluminum base alloy consisting of about 6 per cent copper, 0.05 per cent tin, 0.25 per cent cadmium, and 0.5 per cent bismuth, the balance being aluminum.

2. An aluminum base alloy consisting of about 6 per cent copper, 0.05 per cent tin, 0.25 per cent cadmium, 0.2 per centbismuth and 0.2 per cent lead, the balance being aluminum.

3. A free cutting alloy containing from 3 to 12 per cent of copper, from 0.005 to 0.1 per cent of tin, and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, thallium, cadmium, and bismuth, to improve the machining properties, one of said elementsbeing in all cases cadmium, the balance being substantially alumimun.

4. A free cutting alloy containing from 3 to 12 per cent of copper, from 0.005 to 0.1 per cent of tin, a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, thallium, cadmium, and bismuth,to improve the machining properties, one of said elements being in all cases cadmium, and from 0.05 to 2 per cent of hardening metal from the group composed of molybdenum, vanadium, titanium, tungsten, zirconium, and chromium, the balance being substantially aluminum.

5. An aluminum base alloy consisting of about 6 per cent copper, 0.05 per cent tin, 0.25 per cent cadmium, and 0.5 per cent lead, the balance'being I 

